1. Field of the Invention
The present invention relates to a flow control valve which controls a flow rate of fluid and which is preferably suitable for an idling engine speed control valve (ISCV) for controlling an air flow which bypasses a throttle valve during an idling of an engine.
2. Description of Related Art
One type of known flow control valve may cause a valve leakage at a gap between a valve element and a valve seat when the valve element is seated on the valve seat (when the valve is totally closed) because of dispersion regarding dimensional accuracy and assembly accuracy of each element and the like.
Another type of flow control valve may solve the above problem and may reduce the leakage. As shown in FIG. 1, such the flow control valve has a housing 100, an air passage 110 formed in the housing 100, a valve element 120 which opens and closes the air passage 110, and an electromagnetic actuator 130 and the like. A rubber seat valve 140 is attached to a valve seat on which the valve element 120 is seated. The valve element 120 is lifted from the seat valve 140 by an attractive force of the actuator 130 and opens the air passage 110 when current is supplied to a coil 150 installed in the actuator 130. The valve element 120 is pushed to the seat valve 140 by a spring force of a spring 160 and closes the air passage 110 when current is not supplied to a coil 150.
According to this type of flow control valve, the gap between the valve element 120 and the seat valve 140 may be avoided even if dispersion regarding dimensional accuracy and assembly accuracy of each element occurs, because the seat valve 140 is pushed by the valve element 120 and deforms elastically. Therefore, the leakage may be reduced.
However, if this type of flow control valve is applied to an idling engine speed control valve, the rubber seat valve 140 absorbs gasoline, oil and the like and swells as shown in FIG. 2 because such gasoline, oil or the like may be included in an air flow in the air passage 110. In this case, the position of the valve element 120 shifts to the actuator 130 (to the right in FIG. 1) when the air passage 110 is totally closed. Therefore, the spring force of the spring 160 in a valve closing direction increases, and a required amount of current for the coil 150 to lift the valve element 120 from the full-close position increases. Thus, a predetermined flow rate characteristic (solid line in FIG. 10) changes to the one as shown by the dotted line in FIG. 10.
Another known electromagnetic valve has a valve element which has a spherical seal surface (a surface to be contacted with a valve seat) to prevent a valve leakage caused by a valve element inclination. (See JP-U-4-27277.) According to this electromagnetic valve, a moving core for holding the valve element slides, keeping a contact with a guide portion. Therefore, a gap is formed between the guide portion and the moving core such that the moving core may slide with the contact. If the gap causes a displacement of the moving core in a radial direction for some reason, the center of the spherical seal surface is displaced from a shaft center which is the center of an opening of the valve seat. In this case, the seal surface of the valve element eccentrically contacts with the valve seat, and the valve closes imperfectly.
Furthermore, friction surfaces of the moving core or the guide portion may wear if the valve element is repeatedly seated on or detached from the valve seat. Such wear may cause an expansion of the gap between the guide portion and the moving core, and thereby the moving core is displaced in the radial direction and the center of the spherical seal surface is displaced from the shaft center which is the center of the opening of the valve seat. Therefore, the valve element may not be seated on the valve seat securely (without any gap), whereby valve leakage may result.
Another known flow control valve has been shown in FIG. 3B. A flow control valve 200 is mounted by being embedded in a recess 210 formed in a throttle body 290. The flow control valve 200 has a mounting flange 211. A packing 230 is located between the mounting flange 211 and a mounting surface 220 of the throttle body 290. As shown in FIG. 3A, the flow control valve 200 is screwed and fixed to the throttle body 290 by three screws 222 at the mounting flange 211.
It is desirable to reduce the mounting area of the flow control valve 200 for reducing the size of an engine to reduce the fuel consumption. As shown in FIG. 4, the inventors herein tried a method to screw and fix the flow control valve 200 to the throttle body 290 by one screw 222 at the mounting surface 211, by locating seal members (O-rings) 240 at outer periphery of a housing of the flow control valve 200 and removing the packing 230.
According to such method, tightening force K (FIG. 4) of the screw 222 is applied only at the periphery of the screw 222, and the tightening force K is not applied to the mounting flange 211 at the opposite to the screw 222. Accordingly, as shown in FIG. 5, the mounting flange 211 slid on the mounting surface 220 according to an oscillation in a direction of X, and an oscillation resonance Z, whose oscillation center is at the screw 222, was generated. Measurement data measured at point C in FIG. 5 showing the existence of the oscillation resonance are shown by chain line B in FIG. 16.